Add type to argument of Widget$serialise
Add type to argument of Widget$serialise

This commit explicitly specifies that the argument to serialise should be an object. This would have prevented issue #1320.

SylvainCorlay/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,SylvainCorlay/ipywidgets,jupyter-widgets/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,SylvainCorlay/ipywidgets,SylvainCorlay/ipywidgets